1. Field of the Invention
This invention relates to a natural gas reforming apparatus adapted to reform a gas fuel, such as a natural gas as a fuel by the thermal energy of an exhaust gas, an oxygen eliminating apparatus provided in the natural gas reforming apparatus, and a gas engine adapted to improve a thermal efficiency by turning a gas fuel into a reformed fuel by the natural gas reforming apparatus.
2. Description of the Prior Art
The development of a gas engine, which uses a natural gas as a main fuel, as a cogeneration system has heretofore been promoted. In a cogeneration system, power is taken out as electric energy by a generator, and water is heated into hot water in a heat exchanger by the heat of exhaust gas energy, this hot water being utilized as hot water supply. The conventional engines using a natural gas as a fuel include, for example, the engines disclosed in Japanese Patent Laid-Open Nos. 108865/1994 and 101495/1994.
In the cogeneration type gas engine disclosed in Japanese Patent Laid-Open No. 108865/1994, the temperature of an exhaust gas is reduced by passing the exhaust gas through a turbocharger, an energy recovery unit and a steam generator, and the low-temperature exhaust gas is used for EGR to reduce NOx, the turbocharger being driven by the exhaust gas from the heat insulating gas engine, the generator-carrying energy recovery unit being driven by the exhaust gas from the turbocharger. In this cogeneration type gas engine, the exhaust gas from the energy recovery unit is sent to the steam generator in the heat exchanger, and water is turned into steam in the steam generator, a steam turbine being driven by the steam to recover the steam as electric energy.
When the combustion chambers in a gas engine using a natural as a fuel are formed to a heat insulating structure out of a material, such as a ceramic material, the compression temperature of the air increases higher than a self-ignition temperature of a natural gas, so that an igniter becomes unnecessary. An engine for a high-efficiency cogeneration system can be provided by providing swirl chambers, into which a fuel is introduced, in addition to primary chambers into which the air is introduced, providing control valves between the primary chambers and swirl chambers, and carrying out an operation of the engine by a diesel cycle. When the combustion chambers are formed to a heat insulating structure, the temperature of an exhaust gas from the gas engine becomes as high as not lower than 850.degree. C. The thermal efficiency of the engine can be improved by recovering thermal energy from a high-temperature exhaust gas.
It is known that the main component of a natural gas is methane CH.sub.4. Since CH.sub.4 of a fuel has a high calorific value and exists in large quantities in the natural world, it is expected as a future substitute fuel for petroleum. When CH.sub.4 is thermally decomposed in the presence of CO.sub.2 via a catalyst, CH.sub.4 is converted into CO (carbon monoxide) and H.sub.2 (hydrogen), and a total calorific value of CO and H.sub.2 becomes not lower than that of CH.sub.4, i.e., 1.38 times that of CH.sub.4. When a natural gas is then turned into a reformed fuel by utilizing the thermal energy of a high-temperature exhaust gas from a heat insulating gas engine for the thermal decomposition of CH.sub.4, it becomes possible to increase the calorific value of the reformed fuel, improve the thermal efficiency of the engine, save the resources and minimize the discharge rate of CO.sub.2.
However, when O.sub.2 is contained in a CO.sub.2 -containing separated gas in a case where CO.sub.2 separated from an exhaust gas is used in a natural gas reforming apparatus adapted to convert CH.sub.4 in a natural gas into H.sub.2 and CO by the thermal energy of the exhaust gas in the presence of CO.sub.2, O.sub.2 and H.sub.2 and CO are reacted in a catalyst unit in the natural gas reforming apparatus, and there is a danger of occurrence of a reaction and explosion in a certain case. Therefore, it is necessary that O.sub.2 contained in the CO.sub.2 -contained gas separated from an exhaust gas which is sent to the natural gas reforming apparatus be reduced to as great an extent as possible.
In general, the exhaust gas from a diesel engine contains about 75% of N.sub.2, about 15% of CO.sub.2 and about 10% of O.sub.2 due to an excess air ratio but it does not always have such a content ratio, i.e., the exhaust gas contains more O.sub.2 in some cases. When the exhaust gas is passed through a CO.sub.2 separation film in a CO.sub.2 separator, a CO.sub.2 -containing gas separated from the exhaust gas comes to contain about 90% of CO.sub.2, about 8% of N.sub.2 and about 2% of O.sub.2. Even when a separated gas of this composition is sent to the natural gas reforming apparatus, not so big a problem arises in a usual case but, in some cases, a phenomenon in which such a separated gas contains not less than 5% of O.sub.2 due to the abnormality in the component ratio of the exhaust gas or in the CO.sub.2 separation film. Therefore, when a separated gas containing not less than 5% of O.sub.2 is sent to the natural gas reforming apparatus, H.sub.2 and CO occurring in the apparatus react with O.sub.2 in the separated gas, and explosion occurs, i.e., the apparatus is put in a very dangerous condition.